Vehicle roll control

ABSTRACT

A vehicle roll control system uses two accelerometers ( 38, 40 ) on the vehicle to detect body roll and then filters out higher frequency roll movements as indicative of a rough road surface. The system also monitors vehicle speed, and increases roll stiffness in response to measured increase in vehicle speed, but decreases roll stiffness in response to detected rough road surfaces.

[0001] The present invention relates to active suspension systems forvehicles, and in particular to such systems which include active rollcontrol.

[0002] It is known from U.S. Pat. No. 5,487,006 to provide a vehiclesuspension system in which a single lateral accelerometer is used togive an indication of the nature of the surface over which the vehicleis travelling. However this system suffers from the disadvantage that itcannot clearly distinguish between lateral acceleration of the wholevehicle, and lateral acceleration caused by body roll produced by arough road surface.

[0003] The present invention provides apparatus for detecting theroughness of a surface over which a vehicle is travelling, the apparatuscomprising roll detection means for measuring roll movements of thevehicle, filtering means for detecting high frequency roll movements,and control means arranged to measure the said roughness by measuringthe level of said high frequency roll movements.

[0004] Preferably the roll detection means comprises a pair ofaccelerometers located on the sprung part of the vehicle at differentdistances from the roll axis of the vehicle.

[0005] Preferably the accelerometers are vertically spaced and eachaccelerometer is arranged to detect acceleration in a direction which islateral to the vehicle.

[0006] Preferably the control means is arranged to monitor the measuredacceleration from each of the accelerometers, to produce a roll signaldependent on the difference between the two measured accelerations saidroll signal being indicative of instantaneous vehicle roll.

[0007] Preferably said difference is an integral over time of theinstantaneous difference between the measured accelerations, or thedifference between respective integrals over time of the two measuredaccelerations.

[0008] The present invention further provides a vehicle suspensionsystem including apparatus according to the invention wherein thecontrol means is arranged to control the roll stiffness of the vehicle,and is arranged to reduce the roll stiffness of the vehicle in responseto the detection of increased surface roughness.

[0009] Preferably the system further comprises vehicle speed detectionmeans wherein the control means is arranged to increase the rollstiffness in response to increasing vehicle speed.

[0010] The present invention also provides a vehicle suspension systemcomprising roll control means for controlling the roll stiffness of thevehicle, roughness measuring means for measuring the roughness of thesurface over which the vehicle is travelling, vehicle speed measuringmeans for measuring the speed of the vehicle wherein the roll controlmeans is arranged to increase the roll stiffness if the vehicle speedincreases but to decrease the roll stiffness of the roughness of thesurface over which the vehicle is travelling increases.

[0011] Preferred embodiments of the present invention will now bedescribed by way of example only with reference to the accompanyingdrawings in which:

[0012]FIG. 1 is a diagrammatic representation of a vehicle including asuspension according to an embodiment of the invention,

[0013]FIGS. 2 and 3 are diagrammatic end views of the vehicle of FIG. 1when level and when under roll respectively,

[0014]FIG. 4 is a graph showing the output from sensors in the vehicleof FIG. 1 when travelling on a rough road,

[0015]FIG. 5 is a diagrammatic representation of the rough roaddetection algorithm used in the system of FIG. 1, and

[0016]FIG. 6 is a graph showing the output produced by the algorithm ofFIG. 5.

[0017] Referring to FIG. 1, a vehicle has four wheels 10, 12, 14, 16each mounted on the vehicle body 18. The vehicle has an independentsuspension, each of the wheels being attached to the body 18 which formsthe sprung part of the vehicle, through a suspension arm 20 so that itcan move vertically relative to the body 18. A roll bar 22 is connectedbetween the two rear wheels 14, 16 to control the roll of the rear ofthe vehicle. The roll bar 22 is split in the middle into two halves 22a, 22 b which can be rotated relative to each other by a rotary actuator24 under the control of a control unit 26. This enables vehicle roll tobe controlled actively in response to signals input to the control unitfrom wheel speed sensors 27 and a number of accelerometers which providesignals indicative of the acceleration of parts of the vehicle body invarious directions. A similar roll bar, which is not shown, would alsonormally be connected between the front wheels 10, 12.

[0018] Referring to FIG. 2 the accelerometers mentioned above includetwo lateral accelerometers 28, 30 which are rigidly mounted on thevehicle body 18 and measure lateral acceleration of the vehicle body.The lateral accelerometers 28, 30 are vertically spaced from each other,the lower one 28 being positioned near the roll axis 32 of the vehicle,i.e. the longitudinal axis about which the vehicle tends to rotateduring roll movements. The upper accelerometer 30 is mounted near thetop of the vehicle body 18 further from the roll axis 32. Therefore rollof the vehicle body 18 about the roll axis will cause larger lateralmovements of the upper accelerometer 30 than of the lower 28.

[0019]FIG. 4 shows the signals produced when the vehicle is travellingover rough ground and the body is rolling at relatively high frequenciesof the order of 10 Hz. Under these conditions the upper accelerometer 30will detect relatively high accelerations because the top part of thebody which is relatively for from the roll axis, typically 1.0 to 1.5meters, will be moving through relatively large distances as the bodyrolls about the roll axis. The lower accelerometer 28, on the otherhand, which is closer to the roll axis, say 0.1 or 0.2 meters above it,will detect much lower accelerations because of the smaller lateraldisplacements it will undergo. However it will be understood that, asshown in FIG. 4, the oscillations in the two signals will be in phasewith each other provided the accelerometers are both above the rollaxis. (If the lower accelerometer were below the roll axis they would bein anti-phase.) The signals from the two accelerometers can therefore beanalysed by the control unit to determine the amount of vehicle roll.

[0020] Referring to FIG. 5 in order to determine the roughness of thesurface or road over which the vehicle is travelling, the differencebetween the signals from the upper and lower accelerometers 28, 30 isused to produce a difference signal 50. This signal is then filteredusing a high pass filter 52 and the modulus of the filtered signalproduced at 54. This modulus signal is therefore an indication of theinstantaneous amount of body roll at the high frequencies generallyindicative of a rough surface. These frequencies will depend on the typeof surface which is of interest, and will generally be of the order of10 to 100 Hz.

[0021] A road speed signal 58 which increases with the road speed of thevehicle is passed through a low gain amplifier 60 and a high gainamplifier 62 to produce low gain and high gain speed signals 64, 66. Athreshold device 68 monitors the vehicle speed signal and provides arelatively high threshold signal 70 if the speed signal exceeds apredetermined threshold. A switch unit 72 has an output 74 and connectsthe low gain speed signal to it if the vehicle speed is below apredetermined speed V₁, connects the high gain speed signal to it if thevehicle speed is above the predetermined speed V₁, and permanentlyconnects the output from the threshold device to it.

[0022] The output from the switching device is compared with the modulussignal and the difference input to an integrator 76. The output of theintegrator 76 is, at any time t, the integral over the precedinginterval δ of the difference between the modulus signal and the and theoutput from the switching device.

[0023] The result is that the output R from the integrator 76 willgenerally increase with road roughness, but will decrease with roadspeed as is illustrated in FIG. 6, and the control unit can control theroll stiffness of the vehicle in response. From time t₀ the vehiclestarts off at low speed on a rough surface. This produces a high outputfrom the integrator 76 which causes the control unit 26 to reduce theroll stiffness of the vehicle to allow the suspension to absorb thevibrations produced by the rough surface. At time t₁ the vehicle speedincreases above the predetermined speed V₁ and the output from theintegrator 56 therefore falls. The control unit therefore increases theroll stiffness. At time t₂ the road surface becomes smoother so theoutput from the integrator decreases again, and again the roll stiffnessis increased. At time t₃ the vehicle speed increases above the thresholdspeed. The output from the threshold device is therefore input to theintegrator 56, the output of which falls rapidly. The roll stiffness istherefore rapidly increased as is suitable for higher speed travel onroad.

1. Apparatus for detecting the roughness of a surface over which avehicle is travelling, the apparatus comprising roll detection means formeasuring roll movements of the vehicle, filtering means for detectinghigh frequency roll movements, characterized by control means arrangedto measure said roughness by measuring the level of said high frequencyroll movements.
 2. Apparatus according to claim 1 wherein the rolldetection means comprises a pair of accelerometers located on the sprungpart of the vehicle at different distances from the roll axis of thevehicle.
 3. Apparatus according to claim 2 wherein the accelerometersare vertically spaced and each accelerometer is arranged to detectacceleration in a direction which is lateral to the vehicle. 4.Apparatus according to claim 2 or claim 3 wherein the control means isarranged to monitor the measured acceleration from each of theaccelerometers, to produce a roll signal dependent on the differencebetween the two measured accelerations said roll signal being indicativeof instantaneous vehicle roll.
 5. Apparatus according to claim 3 orclaim 4 wherein said difference is an integral over time of theinstantaneous difference between the measured accelerations. 6.Apparatus according to claim 3 or claim 4 wherein said difference is thedifference between respective integrals over time of the two measuredaccelerations.
 7. A vehicle suspension system comprising apparatusaccording to any foregoing claim for detecting surface roughness whereinthe roll control means is arranged to control the roll stiffness of thevehicle, and is arranged to reduce the roll stiffness of the vehicle inresponse to the detection of increased surface roughness.
 8. A systemaccording to claim 7 further comprising vehicle speed detection meanswherein the control means is arranged to increase the roll stiffness inresponse to increasing vehicle speed.
 9. A vehicle suspension systemcomprising roll control means for controlling the roll stiffness of thevehicle, roughness measuring means for measuring the roughness of thesurface over which the vehicle is travelling, vehicle speed measuringmeans for measuring the speed of the vehicle characterized in that theroll control means is arranged to increase the roll stiffness if thevehicle speed increases but to decrease the roll stiffness of theroughness of the surface over which the vehicle is travelling increases.10. Vehicle suspension apparatus substantially as hereinbefore describedwith reference to the accompanying drawings.